Optical access networks provide optical fiber connections between public networks and end-users. They offer an important solution for the implementation of broadband access networks. They combine a large transport capacity with the capability of serving a wide geographical area. They support connections from central network entities like local exchanges to residential network units and thus can be used to deploy concepts like fiber-to-the-home (FTTH) and fiber-to-the-curb (FTTC). Passive optical networks (PONs) have been studied for many years but, until now, successful commercial deployment was growing slowly.
One possible structure that has been suggested for optical access networks is the point-to-point structure, in which a separate fiber connection is provided between a central office and an optical network unit at each user end-point. This is conceptually simple and it is relatively simple to deal with failures in the fiber connections or in the end-point equipment, since a failure in one connection or one optical network unit does not affect users connected to other optical network units by other connections. However, the point-to-point structure, in some circumstances, suffers from the disadvantage that there may be very many fiber connections terminating in the same central office. For example, for a group of 32 optical network units, there would need to be 32 or 64 fibers connected to the central office, depending on whether the fiber connections were duplex connections, and 32 optical transceivers at the central office, one for each optical network unit.
It has been suggested to provide a single fiber connection (one or two fibers) from the central office to an optical switch situated outside the central office at a suitable location near the user end-points, a so-called “curb switch”, with individual connections between the curb switch and each user's optical network unit. This means that there is only one fiber connection to the central office and one optical transceiver at the central office but, in the case of 32 end-points, the curb switch would need 33 optical transceivers, one for each optical network unit connection and one for the connection to the central office. Also, an optical switch is a sophisticated piece of apparatus, consuming a considerable amount of power and requiring management to monitor and, when necessary, rectify performance, so there are disadvantages in locating such a switch outside the central office. Furthermore, such equipment represents a considerable investment, and levels of use at the outer fringes of the network are likely to be low.
It has also been proposed to use a tree-and-branch topology, in which one fiber connection from an optical line terminal at a central office is connected to a passive optical splitter/combiner, which is in turn connected to optical network units by separate fiber connections. As with the curb switch, one fiber connection to the optical line terminal at the central office serves a number of end-users, but in this case the only equipment situated outside the central office is a passive device requiring little maintenance and no power supply. However, the number of user end-points that can be accommodated is limited because of the use of power splitters; the more optical network units there are, the less the power that reaches each one. Also, for upstream packet transmissions using time domain multiple access, synchronization is required to prevent collisions of packets and loss of data. U.S. Pat. No. 6,470,032, for example, shows a technique for synchronizing the clocks on such a network.
Another known network topology is the ring topology. In a ring network, the network units are connected in a series. There is a one-way connection from a line terminal at the central office to the first network unit in the series, one-way connections from each of the network units to the next in the series, and a one-way connection from the last network unit in the series back to a line terminal at the central office. It is a simple matter to prevent collisions between packets, for example by using a token ring arrangement, such as the IEEE 802.5 standard. Also, each network unit acts as a repeater. However, although the ring topology is useful for computer networks, where all the network units are accessible to central management, it is not currently favored for optical access networks, because the failure of one user's network unit causes a break in the ring and adversely affects all the users on the ring. Also, unauthorized or malicious usage is something that known ring topology networks cannot protect against.